When it's cool enough to touch -- because you have to hold it a bit to unmold it (check out the accompanying slideshow on the right side of the recipe page). At least 30 minutes after it comes out of the oven.

I have made this cake twice (it is really fabulous!), and I have waited each time until the pan was cool enough to handle before I unmolded the cake (probably about 20 or 30 minutes). Each time, the cake was still on the warm side, though not steaming hot.